Biological sciences / 9. Biochemistry and biophysics
O.N. Bliznjuk, A.N. Ogurtsov, N.Yu. Masalitina,
A.S. Savenkov, V.V. Rossikhin
onbliznjuk@ukr.net
PROMOTIVE EFFECT OF CERIUM AND COPPER ON MN-BI MIXED
OXIDE CATALYST
FOR AMMONIA OXIDATION TO N2O
Selective catalytic oxidation of
ammonia with air at low temperatures is an efficient method to produce N2O
as oxidizing agent for organic synthesis. This process has two important
parameters: the selectivity and the application temperature. To rationally
develop a process for NH3 oxidation to N2O over
catalysts, the reaction mechanism must be clarified. While several studies have
examined the low temperature oxidation process, the mechanism of NH3
oxidation and N2O formation is still uncertain. Generally use an imide (NH) mechanism in which the first step yields NH, and
then the NH reacts with atomic oxygen (O) to form nitroxyl
(HNO) and further conversion to N2 or nitrous oxide, or NH could
even react with molecular O2 to produce nitric oxide [1].
In the present paper the influence of
catalyst composition and some operating variables were evaluated by IR-spectroscopy
in terms of N2O formation, using Mn/Bi/Cu/Ce-oxide
catalysts.
Some single and multi-oxides used for active phases were synthesized by
sol-gel method. Different from precipitation, solid-state reaction or spray
drying, this method is based on the addition of an organic complexation
agent (here citric acid) into the precursors. The presence of the organic complexation agent distinguishes this complexation
method from the other methods owing to the complexation
and gelation steps. These steps are influenced mainly
by the atomic ratio of citric acid to metal cations
and pH of the solution. Sol-gel method leads to the formation of very pure and
homogeneous catalyst powders exhibiting high surface area. Different salts of Mn(NO3)2, Cu(NO3)2×6H2O, Ce(NO3)3×6H2O, Bi(NO3)2×4H2O were dissolved in water in order to obtain the solution
with the concentration of 0.125M. 10%wt. citric acid solutions prepared from
citric acid monohydrate – C6H8O7×H2O.
MnO2-Bi2O3-CuO-CeO2
catalyst was synthesized by dropping a suitable amount of Cu(NO3)2,
Bi(NO3)3 and Ce(NO3)3
solutions into a suitable volume of Mn(NO3)2
solution corresponding to different MnO2/Bi2O3/CuO/CeO2
molar ratios. If precipitation occurred, concentrated HNO3 solution
was added until the precipitates disappear. A suitable amount of citric acid
solution was dropped into the obtained solution with the molar ratio of citric to
metals of 2. The obtained solution was stabled within 30 minutes and evaporated
at 70–80ºC until the gel was obtained. The gel was then dried at 110ºC
for 3 hours. The obtained solid were calcinated at
550ºC for 3 hours with the heating rate is 3ºC/min.
Single metallic oxides, bi-metallic oxides, other
triple metallic oxides and tetra metallic oxides were synthesized similarly. The catalysts were characterized by some techniques,
such as: X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission
Electron Microscopy (TEM), BET for detemining
specific surface area, Xray Photoelectron Spectrocopy (XPS), TG-DTA, TG-DSC, Infrared Spectrocopy (IR). Catalysis activity of the catalysts were examinied in a micro reactor set up.
The IR spectra of ammonia adsorbed on
the catalysts show the bands at 1594 and 1165 cm–1, which attributed
to σas and σs
model of NH3 coordinated to Lewis acid sites. Another two bands at
1674 and 1445 cm–1 are attributed to σs
NH4+ and σas NH4+
resulting from ammonia coordinated to Brønsted
acid sites [2]. It is indicated by the increase in intensity of band at 1165 cm–1
that more Lewis acid sites are generated on Mn/Bi/Cu/Ce-oxide
by introduction of Cex+ which
can also serve as Lewis acid sites. Comparison of IR spectra from catalyst
treated with 1000 ppm NO, 1000 ppm
NO2, and 1000 ppm NO + 2% O2
shows the five bands at 1610, 1550, 1466, 1291, and 1030 cm–1. The
bands at 1550, 1291, and 1030 cm–1 can be assigned to bidentate nitrate; the band at 1466 cm–1 can be
attributed to the monodentate nitrite [3]. The
mechanism proposed for N2O generation at low temperature is based on
the formation of surface Ce-ON species which may be
produced by the partial oxidation of dissociatively
adsorbed ammonia species with NO + O2 (eventually NO2)
[4]. When these active sites are in close proximity they can interact to form
an N2O molecule.
[1] N.Yu.
Masalitina, A.S. Savenkov,
O.N. Bliznjuk, A.N. Ogurtsov,
[2] W.U.
Xiaodong, S.I. Zhichun,
L.I. Guo, W. Duan, J. Rare Earth 29 (2011) 64.
[3] G.
Qi, R.T. Yang, J.
Phys. Chem. B
108 (2004) 15738.
[4] J.A. Martin, M. Yates, P. Avila,
S. Suares, J. Blanco, Appl. Catal. B Environ.
70 (2007) 330.